Image sensor, lithographic apparatus comprising an image sensor and use of an image sensor in a lithographic apparatus

ABSTRACT

The invention relates to an image sensor for detection of an aerial image formed by a beam of radiation in a lithographic projection apparatus for exposing a pattern onto a substrate held in a substrate plane by a substrate holder. The image sensor has an image detector and a lens. The lens is arranged to project at least part of the aerial image onto the image detector. The image sensor is positioned such within the substrate holder that the lens is positioned proximate the substrate plane.

FIELD

The present invention relates to an image sensor, a lithographicapparatus comprising an image sensor, patterning device for use in sucha lithographic apparatus, use of an image sensor in a lithographicapparatus and computer program product enabling such a use.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In device manufacturing methods using lithographic apparatus, animportant factor in the yield, i.e. the percentage of correctlymanufactured devices, is the accuracy within which layers are printed inrelation to layers that have previously been formed. This is known asoverlay and the overlay error budget will often be 10 nm or less. Toachieve such accuracy, the substrate must be aligned to the mask patternto be transferred with great accuracy.

A number of sensors is used at substrate level for evaluating andoptimizing imaging performance. These may include transmission imagesensors (TIS). A TIS is a sensor that is used to measure at substratelevel the position of a projected aerial image of a mark pattern at mask(reticle) level. The projected image at substrate level may be a linepattern with a line width comparable to the wavelength of the exposureradiation. The TIS measures aforementioned mark pattern using atransmission pattern with a photocell underneath it. The sensor data maybe used to measure the position of the mask with respect to thesubstrate table in six degrees of freedom, i.e. three degrees of freedomrelated to translation and three degrees of freedom related to rotation.Moreover, magnification and scaling of the projected mark pattern may bemeasured. With a small line width, the sensor is capable of measuringthe pattern positions and influences of several illumination settings,e.g. annular, dipole, for several mask types (binary mask, phase-shiftmask). The TIS may also be used to measure optical performance of atool, like a lithographic projection apparatus. By using differentillumination settings in combination with different projected images,properties such as pupil shape, coma, spherical aberration, astigmatismand field curvature can be measured.

With the continual desire to image ever smaller patterns to createdevice with higher component densities, there is pressure to reduceoverlay errors, which leads to a desire for improved sensors. Moreover,aforementioned ever smaller patterns require more often than beforecritical device structures in the mask pattern which substantiallydiffer from the mark pattern used. The critical device structures followa different transmission path than the mark pattern, and, as a result,encounters different aberrations along its transmission path.Deformations formed as a result of the different transmission path maylead to overlay and focus errors.

SUMMARY

It is desirable to provide a sensor at substrate level with highsensitivity that can be used in high NA systems, i.e. immersionlithographic apparatus, and capable of measuring critical structures.

To that end, the invention provides an image sensor for detection of anaerial image formed by a beam of radiation in a lithographic projectionapparatus for exposing a pattern onto a substrate held in a substrateplane by a substrate holder, the image sensor comprising

an image detector; and

a lens arranged to project at least part of the aerial image onto theimage detector; wherein the image sensor is positioned such within thesubstrate holder that the lens is positioned proximate the substrateplane.

Additionally, in an embodiment, the invention provides a method fortransmission image detection of an aerial image formed in a lithographicprojection apparatus by radiation of a predetermined wavelengthcomprising:

providing a structure provided with an object mark;

providing such an image sensor;

providing a projection system between the structure and the detector

forming an object mark pattern upon illumination of the object mark bythe radiation with a predetermined wavelength;

forming an object mark aerial image of the object mark by the projectionsystem at an image side of the projection system;

detecting the aerial image with the image sensor.

Additionally, in an embodiment, the invention provides a lithographicapparatus comprising such an image sensor, the sensor beingsubstantially positioned in the substrate plane.

Additionally, in an embodiment, the invention provides a devicemanufacturing method comprising projecting a patterned beam of radiationonto a substrate using such a lithographic apparatus.

Additionally, in an embodiment, the invention provides a patterningdevice for use in such a lithographic apparatus, the patterning devicebeing provided with a proximity curve mark.

Additionally, in an embodiment, the invention provides a patterningdevice for use in such a lithographic apparatus, the patterning devicebeing provided with a critical dimension product feature.

Additionally, in an embodiment, the invention provides a use of such alithographic apparatus to align the patterning device supported by thesupport structure with respect to the substrate holder.

Additionally, in an embodiment, the invention provides a computerprogram product comprising computer executable code, which, when loadedon a computer assembly, enables the computer assembly to control such ause.

Additionally, in an embodiment, the invention provides a use of such alithographic apparatus, the lithographic apparatus further beingprovided with such a patterning device, to determine a proximity curvecharacteristic for the lithographic apparatus.

Additionally, in an embodiment, the invention provides a computerprogram product comprising computer executable code, which, when loadedon a computer assembly, enables the computer assembly to control such ause.

Additionally, in an embodiment, the invention provides a use of such alithographic apparatus, the lithographic apparatus further beingprovided with a such a patterning device, to determine optimalillumination conditions of the lithographic apparatus.

Additionally, in an embodiment, the invention provides a computerprogram product comprising computer executable code, which, when loadedon a computer assembly, enables the computer assembly to control such ause.

Additionally, in an embodiment, the invention provides a use of such alithographic apparatus to verify optical proximity correction.

Additionally, in an embodiment, the invention provides a computerprogram product comprising computer executable code, which, when loadedon a computer assembly, enables the computer assembly to control such ause.

Additionally, in an embodiment, the invention provides a use of such alithographic apparatus to determine an interrelationship betweenaberrations of the projection system and the aerial image of thepattern.

Finally, in an embodiment, the invention provides a computer programproduct comprising computer executable code, which when loaded on acomputer assembly, enables the computer assembly to control such a use.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 schematically depicts an arrangement of the substrate tabledepicted in the lithographic apparatus of FIG. 1 provided with imagesensors according to an embodiment of the invention;

FIG. 3 schematically depicts a cross-section of part of a lithographicapparatus comprising an embodiment of an image sensor according to thepresent invention;

FIG. 4 schematically depicts an embodiment of an image sensor accordingto the present invention;

FIG. 5 schematically depicts an embodiment of a lens used in anembodiment of an image sensor according to the present invention;

FIG. 6 schematically depicts an arrangement for use of embodiments of animage sensor according to the present invention;

FIG. 7 schematically depicts a mask provided with marks which may beimaged by embodiments of an image sensor according to the presentinvention.

FIG. 8 schematically depicts an embodiment of a computer assembly thatmay be used by an arrangement for use of embodiments of an image sensoraccording to the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or EUV-radiation).

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam B, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 schematically depicts an arrangement of the substrate table WTdepicted in the lithographic apparatus of FIG. 1, in which the substratetable WT is provided with an embodiment of the image sensor according tothe invention. In the substrate table WT, two image sensors IAS1 andIAS2 are provided. The image sensors can be used to determine a locationof an aerial image of a pattern, e.g. an object mark, on the mask MA byscanning the image sensor IAS1 or IAS2 through the aerial image.

By means of the image sensors IAS1 and IAS2, when their position in thesubstrate table is well-known, the relative position of the aerial imageof the pattern on the mask MA with respect to the substrate table WT canbe determined. The substrate table WT may be provided with a substrate Wcomprising substrate marks, e.g. substrate marks P1, P2, P3, P4 asdepicted in FIG. 2. An alignment sensor (not shown) may previouslyobtain relative positions of the substrate marks P1, P2, P3, P4. Theknowledge of the relative positions of the substrate marks P1, P2, P3,P4 obtained by the alignment sensor combined with the relative positionof the image of the object mark on the mask MA with respect to the wafertable WT deduced from information obtained with the image sensors IAS1,IAS2, allow the substrate W to be positioned at any desired positionrelative to the projected image of the mask MA with great accuracy. Itmust be understood that instead of two image sensors IAS1 and IAS2, moreor less image sensors may be present, e.g. one or three.

FIG. 3 schematically depicts a cross-section of part of a lithographicapparatus comprising an embodiment of an Image sensor according to thepresent invention. The cross-section shows a final element FE of theprojection system PL positioned on top of an image sensor 1 embedded inthe substrate table WT.

The embodiment of the image sensor 1 depicted in FIG. 3 is positioned inan immersion lithographic apparatus. In the immersion arrangement shownin FIG. 3, a reservoir 3 forms a contactless seal to the wafer table WTprovided with the image sensor 1 around an image field of the projectionsystem PL so that liquid is confined to fill a space between the surfaceof the substrate table WT provided with the image sensor 1 and the finalelement FE of the projection system PL.

The image sensor 1 comprises a lens 5 and an image detector 6. The lens5 is arranged to project at least part of an aerial image of a pattern,schematically depicted by the dotted lines, projected on the lens 5 bymeans of the final element FE of the projection system PL, on the imagedetector 6. The image detector 6 comprises a detecting surface. Thedetecting surface may be constructed in a matrix form, such that thedetecting surface is composed of a plurality of pixels. The imagedetector 6 may be a CCD-camera or a CMOS-camera. The lens 5 may be amicroscope lens. The lens 5 may have a magnification between 1500 and2500.

FIG. 4 schematically depicts an embodiment of an image sensor 1according to the present invention. In this embodiment, the imagesensor, next to the lens 5 and image detector 6, further comprises anamplification device 8 which is positioned between the lens 5 and theimage detector 6. In an embodiment, the amplification device is amultichannel plate.

The amplification device 8 may be mounted on the detector, e.g. in a wayshown in FIG. 4, or, alternatively, be positioned in close proximitythereof. In yet another embodiment, the amplification device 8 isintegrated in the image detector 6, e.g. a plurality of avalanche diodesis arranged such that each avalanche diode of the plurality of avalanchediodes corresponds with a single pixel of the image detector 6.

The amplification device 8 is arranged for amplifying the incoming lightintensity. As a result, more light falls on the detection surface of theimage detector 6, which may improve its imaging performance. The image,schematically depicted by the dotted lines, that is detected by means ofthe image detector 6 may be transferred in form of an information signal10 towards a processor, e.g. a processor used in a computer assembly asshown in FIG. 7.

FIG. 5 schematically depicts the lens 5 in an embodiment of the imagesensor according to the present invention in more detail. The lens 5 isintegrated in the substrate table WT. At its top surface, i.e. thesurface facing the incoming light which in a lithographic apparatuscorresponds with the surface facing the final element FE of theprojection system PL, the lens 5 is provided with at least one lensreference mark 11. By means of the at least one lens reference mark 11on the top surface of the lens 5, the position of the lens 5 withrespect to the substrate table WT may be determined.

FIG. 6 schematically depicts an arrangement for use of embodiments of animage sensor 21 according to the present invention. At the left, severalelements of a lithographic apparatus are shown, i.e. a mask MA and aprojection system PL. The mask MA is configured to impart an incomingbeam of radiation with a pattern in its cross-section. The projectionsystem PL is configured to expose the patterned beam on a substrate (notshown). In case a measurement is taken with an embodiment of the imagesensor 21, the projection system PL instead exposes the patterned beamon the image sensor 21. The arrangement further comprises a control unit23 and a parameter adjustment device 25. The control unit 23 isoperationally coupled to the image sensor 21 and the parameteradjustment device 25, and may also be operationally coupled to otherelements of a lithographic apparatus, e.g. substrate table WT and masktable MT.

The image sensor 21 is arranged to transfer image data to the controlunit 23. The control unit 23 in its turn, is arranged for receiving theimage data from the image sensor 21. In response, the control unit 23may control a parameter of the lithographic apparatus, e.g. by changingsettings of the parameter adjustment device 25, altering a position ofthe substrate table WT or altering a position of the mask MA or masktable MT.

The control unit 23 may comprise a processor 27 and a memory 29. Furtherdetails with respect to arrangements of a control unit are explainedwith reference to FIG. 8.

The arrangement as depicted in FIG. 6 may be used for several purposes.In the following paragraphs several uses are described. The descriptionof the uses is intended to be illustrative, not limiting. Thus, it willbe apparent to one skilled in the art that different uses of thearrangement remain possible without departing from the scope ofinvention.

Use to Align Substrate Table with Respect to Mask (Table)

An embodiment of the arrangement may be used in a similar way as thetransmission image sensor TIS that is used in state-of-the-artlithographic machines, i.e. to determine and correct the position of thesubstrate table WT and the substrate W residing thereon with respect tothe mask table MT or, alternatively, to mask MA. However, asschematically depicted in FIG. 7, instead of a specially designed objectmark 31 residing on the mask MA as depicted in FIG. 7, or,alternatively, on the mask table MT, which specially designed objectmark 31 typically has a size of 64×40 microns at substrate level, a mark33 may be used of a much smaller size, e.g. 1×1 microns at substratelevel. The mark 33 comprises critical patterns, i.e. patterns with ashape and dimension typical for the pattern to be exposed on thesubstrate W.

Aberrations in the projection system PL may be entirely different forfeatures of different sizes, and may also differ spatially, i.e. theaberration is different for light passing the projection system PL at afirst location as compared to light passing the projection system PL ata second location. As the features of the marks 33 are of the samedimensions as the pattern to be exposed, the observed aberrations give abetter impression of the aberrations that will be encountered by themask pattern during exposure.

Moreover, as the marks 33 that can be used by embodiments of the imagesensor 1, 21 do not occupy a lot of space, i.e. a few squared microns atsubstrate level at the most, the marks 33 may be present within theexposure area of the mask MA, schematically depicted as dotted square 37in FIG. 7. On the other hand, the specially designed object mark 31 thatis suitable for a conventional TIS-sensor would be positioned at an edgeof the mask MA, i.e. outside the exposure area. As can be seen in FIG.7, within an exposure area 37, several dies, denoted by the squares 39,may be present. Each die 39 may be provided with a different pattern.The marks 33 may be present within a die 39, e.g. marks 33 a and 33 b.Additionally or alternatively, the marks 33 may be present between thedies 39, e.g. marks 33 c, 33 d and 33 e.

Finally, aforementioned possibility opens the possibility to use a smallpattern of the actual product feature(s) to be exposed. The sensor canbe used without the use of a dedicated mark.

An image of the mark 33 suitable for embodiments of the image sensoraccording to the invention follows an optical path through theprojection system PL which is more similar to the optical path that isfollowed by the pattern on the mask MA to be exposed on the substrate W.Consequently, a position of the substrate table WT, and the substrate Wlying thereon with respect to the mask table MT or, alternatively, maskMA may be optimized to an extent beyond present-day capabilities.

Note that it may be possible to use an embodiment of an image sensoraccording to the present invention besides a conventional image sensor,e.g. the aforementioned TIS-sensor. For example, in FIG. 2, IAS1 may bea conventional image sensor, e.g. the aforementioned TIS-sensor, andIAS2 an image sensor in accordance with an embodiment of the presentinvention.

Use to Measure Proximity Curve and Optimize Illumination Settings inResponse

The image sensor 21 in the arrangement may be used to measure aproximity curve, i.e. a curve qualifying the influence of neighboringfeatures on the imaging of a certain feature. In a proximity curve, thechange in printed resist critical dimension is measured for a certaintype of structure, e.g. lines with a specific diameter like 130 nm, on acertain type of mask, e.g. a binary mask, with varying pitches, e.g.ranging from 1:1, i.e. space between lines is equal to line width, toisolated lines in a certain number of steps, e.g. ten. Such a range maybe provided in the form of a proximity curve mark.

A proximity curve is generally machine-dependent. Information regardingthe proximity curve measured by the image sensor 21, e.g. by measuringon a proximity curve mark, may be compared by the control unit 23 withproximity curves from other machines, e.g. by using the processor 27 inthe control unit 23 to compare the measurement results with proximitycurves of other machines stored as reference data in memory 29 ofcontrol unit 23. Additionally, or alternatively, the measured proximitycurve may serve as an input for the processor 27 of the control unit 23to determine in what way and to what extent parameters should be changedto obtain optimal exposure results. In response to reception of theproximity curve, the processor 27 of the control unit 23, optionally byusing information stored in memory 29 of control unit 23, calculatesadjustment data to adjust at least one parameter in the lithographicapparatus, e.g. the illumination settings.

The adjustment data are transferred towards the parameter adjustmentdevice 25. In an embodiment, the parameter adjustment device 25comprises an array, e.g. more than 1000, reflective elements, thereflective elements being arranged in a grid-like formation andindividually controllable with respect to their orientation. In anembodiment, the parameter adjustment device 25 is an illuminationsettings adjustment device. Possible adjustments related to illuminationsettings include adjustments leading to a change of the numericalaperture NA of the projection system PL and adjustments of the angulardistribution of light falling on the mask, also referred to as σ. Inillumination settings for angular illumination, the angular distributionof the outer light cone, i.e. σ_(out), and the inner cone σ_(in) may bealtered separately.

In an embodiment it is possible to measure aforementioned proximitycurve on-line. Consequently, the illumination settings may be adjustedon a substrate-to-substrate basis to obtain so calledsubstrate-to-substrate proximity control.

Use to Measure critical CD on Product Features and Optimize SourceConditions in Response

Instead of a proximity curve, critical dimensions (CD) on productfeatures which are critical for a certain product development may bemeasured by image sensor 21. The processor 27 of the control unit 23,optionally in collaboration with a memory like memory 29 of the controlunit 29, may calculate parameter adjustment data, upon reception of themeasurement results on the critical CD on product features. In thiscase, a parameter to be adjusted may again be an illumination settingincluding adjustments related to changes in NA or adjustments related tochanges of σ. The parameter adjustment device 25 may again comprise anarray of reflective elements, and may again be an illuminationadjustment device positioned in close proximity of the mask MA or masktable MT between the source (not shown) and the mask MA or mask tableMT, as described earlier.

Alternatively or additionally, adjustments may relate to changing thetype of illumination by adjustment of the source that is used, e.g. froma dipolar illumination setting towards an annular illumination settingor from a first annular illumination setting towards a second annularillumination setting. In these cases, the parameter adjustment device 25is a source adjustment device. The source adjustment device may directlyadapt parameters with respect to the source. Also in this case, theparameter adjustment device 25, in an embodiment, may comprise an arrayof reflective elements as described earlier.

Adjustments are not limited to changes of the type of illumination. Thetype of illumination may remain the same while a property of that typeof illumination is adjusted. For example, the illumination may bestretched in a particular direction, may be made larger, made smaller,etc.

In an embodiment it is possible to measure critical dimensions on-line.Consequently, the illumination settings may be adjusted on asubstrate-to-substrate basis to obtain substrate-to-substrateillumination setting optimization.

Use to Perform On-Line Optical Proximity Correction (OPC) Verification

The image sensor 21 may be used to verify whether additional structuresprovided in the pattern of a mask MA for the purpose of OPC, i.e. tocontrol the shapes of desired pattern structures, are positioned at theright location in the pattern. The position of the additional structuresmay be determined and the processor 27 of the control unit 23 may usethe determined position to calculate, optionally by using reference datastored in memory 29 of the control unit 23, the effect of the additionalstructure on the main structure after exposure and development inresist. If the position is wrong, and the additional structure providesan undesired change of the shape of the desired main pattern structure,the mask may be replaced or improved before real exposure on resisttakes place.

Use to Investigate Relation Aerial Image and Aberration Fingerprint

Currently, models are used to simulate what changes occur in an aerialimage as a result of a change in aberrations of the projection systemPL. With an arrangement as shown in FIG. 6, an investigation of therelation between the aerial image and the aberration fingerprint of theprojection system PL may be performed. For this purpose, a wavefrontaberration sensor 31 is added. The wavefront aberration sensor 31, e.g.an interferometric wavefront measurement system like a so-calledIntegrated Lens Interferometer At Scanner (ILIAS), may be used tomeasure the aberration fingerprint of the projection system PL on thesurface of the substrate that is placed on the substrate table WT.Simultaneously, an aerial image may be observed by means of the imagesensor 21. The wavefront aberration sensor 31 is configured to transferinformation with respect to the aberration fingerprint towards thecontrol unit 23. The processor 27 of the control unit 23 is configuredto compare the aberration fingerprint information originating from thewavefront aberration sensor 31 with the image data obtained from theimage sensor 21. Upon comparison, several trends may be derived,optionally by using data stored in memory 29 of the control unit 23. Asa result, one can for example monitor structurally dependent offsets ofaberrations.

It should be understood that the control unit 23 may be a computerassembly 60 as shown in FIG. 8. The computer assembly 60 may be adedicated computer in the form of a control unit in embodiments of theassembly according to the invention or, alternatively, be a centralcomputer controlling the lithographic projection apparatus. The computerassembly 60 may be arranged for loading a computer program productcomprising computer executable code. This may enable the computerassembly 60, when the computer program product is downloaded, to controlaforementioned uses of a lithographic apparatus with embodiments of theimage sensor.

The memory 29 connected to processor 27 may comprise a number of memorycomponents like a hard disk 31, Read Only Memory (ROM) 62, ElectricallyErasable Programmable Read Only Memory (EEPROM) 63 en Random AccessMemory (RAM) 64. Not all aforementioned memory components need to bepresent. Furthermore, it is not essential that aforementioned memorycomponents are physically in close proximity to the processor 27 or toeach other. They may be located at a distance away

The processor 27 may also be connected to some kind of user interface,for instance a keyboard 65 or a mouse 66. A touch screen, track ball,speech converter or other interfaces that are known to persons skilledin the art may also be used.

The processor 27 may be connected to a reading unit 67, which isarranged to read data, e.g. in the form of computer executable code,from and under some circumstances store data on a data carrier, like afloppy disc 68 or a CDROM 69. Also DVD's or other data carriers known topersons skilled in the art may be used.

The processor 27 may also be connected to a printer 70 to print outoutput data on paper as well as to a display 71, for instance a monitoror LCD (Liquid Crystal Display), of any other type of display known to aperson skilled in the art.

The processor 27 may be connected to a communications network 72, forinstance a public switched telephone network (PSTN), a local areanetwork (LAN), a wide area network (WAN) etc. by means oftransmitters/receivers 73 responsible for input/output (I/O). Theprocessor 27 may be arranged to communicate with other communicationsystems via the communications network 72. In an embodiment of theinvention external computers (not shown), for instance personalcomputers of operators, can log into the processor 27 via thecommunications network 72.

The processor 27 may be implemented as an independent system or as anumber of processing units that operate in parallel, wherein eachprocessing unit is arranged to execute sub-tasks of a larger program.The processing units may also be divided in one or more main processingunits with several subprocessing units. Some processing units of theprocessor 27 may even be located a distance away of the other processingunits and communicate via communications network 72.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled person will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” and “target portion”, respectively. The substrate referredto herein may be processed before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. Image sensor for detection of an aerial image formed by a beam ofradiation in a lithographic projection apparatus for exposing a patternonto a substrate held in a substrate plane by a substrate holder, theimage sensor comprising an image detector; and a lens arranged toproject at least part of the aerial image onto said image detector;wherein said image sensor is positioned such within said substrateholder that said lens is positioned proximate said substrate plane. 2.Image sensor according to claim 1, wherein said image detector is aCCD-camera or a CMOS-camera.
 3. Image sensor according to claim 1,wherein said lens is a microscope lens.
 4. Image sensor according toclaim 3, wherein said microscope lens has a magnification between 1500and
 2500. 5. Image sensor according to claim 1, wherein said imagesensor further includes an amplification device positioned between saidlens and said image detector.
 6. Image sensor according to claim 5,wherein said amplification device is a multichannel plate.
 7. Imagesensor according to claim 1, wherein said lens comprises reference marksfor determining a position of said image sensor with respect to saidsubstrate table.
 8. Image sensor according to claim 1, wherein said lenshas a numerical aperture larger than 1.2.
 9. Lithographic apparatuscomprising: an illumination system configured to provide a beam ofradiation; a support structure configured to support a patterning devicethat serves to impart said beam of radiation with a pattern in itscross-section; a substrate holder configured to hold a substrate in asubstrate plane; a projection system configured to expose said patternedbeam on said substrate; wherein said lithographic apparatus furthercomprises an image sensor according to claim 1, said image sensor beingsubstantially positioned in said substrate plane.
 10. Lithographicapparatus according to claim 9, wherein said image sensor is arrangedfor generating image data and said lithographic apparatus furthercomprises: a control unit operationally coupled to said image sensor andarranged for receiving said image data from said image sensor, saidcontrol unit comprising a processor for calculating adjustment data; anda parameter adjustment device operationally coupled to said control unitand arranged for controlling at least one parameter of said lithographicapparatus in response to reception of said adjustment data from saidcontrol unit.
 11. Lithographic apparatus according to claim 10, whereinsaid control unit further comprises a memory, said memory arranged forstoring reference data and being accessible for said processor. 12.Lithographic apparatus according to claim 10, wherein said parameteradjustment device is an illumination setting adjustment device arrangedfor adjusting illumination settings of said beam of radiation. 13.Lithographic apparatus according to claim 12, wherein said at least oneparameter is related to illumination settings of said beam of radiationand selected from a group consisting of numerical aperture of theprojection system, angular distribution of said beam of radiation, andtype of illumination provided by a source arranged for generatingradiation for use in said beam of radiation.
 14. Lithographic apparatusaccording to claim 10, wherein said lithographic apparatus furthercomprises a wavefront aberration sensor enabling simultaneousmeasurement of wavefront aberrations of the projection system by meansof said wavefront aberration sensor and measurement of an aerial imageof the pattern by means of said image sensor.
 15. Method fortransmission image detection of an aerial image formed in a lithographicprojection apparatus by radiation of a predetermined wavelengthcomprising: providing a structure provided with an object mark;providing an image sensor according to claim 1; providing a projectionsystem between said structure and said detector forming an object markpattern upon illumination of said object mark by said radiation with apredetermined wavelength; forming an object mark aerial image of saidobject mark by said projection system at an image side of saidprojection system; detecting said aerial image with said image sensor.16. A device manufacturing method comprising projecting a patterned beamof radiation onto a substrate using the lithographic apparatus asdefined by claim
 9. 17. Patterning device for use in a lithographicapparatus according to claim 10, wherein said patterning device isprovided with a proximity curve mark.
 18. Patterning device for use in alithographic apparatus according to claim 10, wherein said patterningdevice is provided with a critical dimension product feature. 19.Patterning device according to claim 17, wherein said critical dimensionproduct feature is located in an area arranged for exposure on saidsubstrate.
 20. Use of lithographic apparatus according to claim 9 toalign said patterning device supported by said support structure withrespect to said substrate holder.
 21. Computer program productcomprising computer executable code, which when loaded on a computerassembly, enables the computer assembly to control the use of alithographic apparatus according to claim
 20. 22. Use of lithographicapparatus according to claim 10, the lithographic apparatus furtherbeing provided with a patterning device according to claim 17, todetermine a proximity curve characteristic for said lithographicapparatus.
 23. Computer program product comprising computer executablecode, which, when loaded on a computer assembly, enables the computerassembly to control the use of a lithographic apparatus according toclaim
 22. 24. Use of lithographic apparatus according to claim 10, thelithographic apparatus further being provided with a patterning deviceaccording to claim 18, to determine optimal illumination conditions ofsaid lithographic apparatus.
 25. Computer program product comprisingcomputer executable code, which, when loaded on a computer assembly,enables the computer assembly to control the use of a lithographicapparatus according to claim
 24. 26. Use of lithographic apparatusaccording to claim 11 to verify optical proximity correction. 27.Computer program product comprising computer executable code, which,when loaded on a computer assembly, enables the computer assembly tocontrol the use of a lithographic apparatus according to claim
 26. 28.Use of lithographic apparatus according to claim 14 to determine aninterrelationship between aberrations of the projection system and theaerial image of the pattern.
 29. Computer program product comprisingcomputer executable code, which, when loaded on a computer assembly,enables the computer assembly to control the use of a lithographicapparatus according to claim 28.